Page 205 - Principles of Applied Reservoir Simulation 2E

1 90 Principles of Applied Reservoir Simulation

fractional changes to the independent variables {Xj} . The fractional change in
F is given by
dF dX

This lets us compare the relative importance of changes to the independent
variables. Notice that the proportionality constant K has been factored out of the
expression dF/Ffor the fractional change in F. Thus, the quantity dF/Fdoes not
depend on the system of units used in the sensitivity study.

19.4 Economic Analysis

In addition to providing technical insight into fluid flow performance,
model predictions are frequently combined with price forecasts to estimate how
much revenue will be generated by a proposed reservoir management plan. The
revenue stream is used to pay for capital and operating expenses, and the
economic performance of the project depends on the relationship between
revenue and expenses [see, for example, Bradley and Wood, 1 994; Mian, 1 992;
Thompson and Wright, 1 985]. A discussion of basic economic concepts is given
in Chapter 9. It is sufficient to note here the role of economic analysis in the
context of a model study.
In a very real sense, the reservoir model determines how much money will
be available to pay for wells, compressors, pipelines, platforms, processing
facilities, and any other items that are needed to implement the plan represented
by the model. For this reason, the modeling team may be expected to generate
flow predictions using a combination of reservoir parameters that yield better
recoveries than would be expected if a less "optimistic" set of parameters had
been used. The sensitivity analysis is a useful process for determining the
likelihood that a set of parameters will be realized. Indeed, modern reserves
classification systems are designed to present reserves estimates in terms of their
probability of occurrence. A probabilistic analysis is discussed in Chapter 9. The
probabilistic representation of forecasts gives decision-making bodies such as